office maintains a paper copy of a file that is the result of a field inspection by an assessor sometime during the course of the history of the building. they went out and took pictures and recorded. this is a snapshot of time, recording the area, the number of rooms, the number of sinks, and so on. that is typically on file at the assessor reporter's office and is updated by them when there is a permit or some other change. there are also many other sources of information that can help us understand changes to utilities -- power companies, utility companies. in this case, where we have an addition to the back of a building, many buildings in san francisco develop these incremental additions where they start as a porch or a washroom or laundry and gradually get in close. next thing you know, you have an addition. it has been very common.